The present invention relates to a semiconductor device and a method for fabricating the semiconductor device, more specifically a semiconductor device using silicon germanium and a method for fabricating the semiconductor device.
Recently, HBT (Heterojunction Bipolar Transistor) using silicon germanium is noted as a device applicable to portable telephones, optical transmission, etc., which require extra-high frequency operations.
The proposed HBT will be explained with reference to FIGS. 10A to 11C. FIGS. 10A to 11C are sectional views of the proposed HBT in the steps of the method for fabricating the HBT, which show the method.
As shown in FIG. 10A, an n+ type buried diffused layer 112 and an n type collector layer 114 are formed sequentially on a p type silicon substrate 110.
Then, a pad oxide film 118 is formed on the surface of the collector layer 114.
Next, an element isolation film 122 is formed by local oxidation. Then, the pad oxide film 118 in a collector 123 is removed.
Then, as shown in FIG. 10B, a polycrystal silicon layer 125 is formed on the entire surface. Next, boron is implanted in a part of the polycrystal silicon layer 125, which is to be an outgoing base electrode 126.
Then, an insulation film 128 of SiN is formed on the entire surface.
Next, an insulation film 129 of SiO2 is formed on the entire surface.
Next, as shown in FIG. 10C, an opening 130 is formed down to the pad oxide film 118 by photolithography.
Next, an insulation film 131 of SiN is formed on the inside wall of the opening 130.
Then, as shown in FIG. 10D, with the insulation film 128 and the insulation film 131 as a mask, the pad oxide film 118, and the insulation film 129 on the insulation film 128 are selectively etched. At this time, the pad oxide film 118 immediately below the outgoing base electrode 126 is also etched.
Then, a shown in FIG. 11A, single crystal p type base layer 132 of silicon germanium is grown in a region where the collector layer 114 is exposed. A p type dopant implanted in the base layer 132 is boron.
Next, as shown in FIG. 11B, an insulation film 134 of SiO2 and an insulation film 135 of SiN are formed sequentially on the inside of the opening 130 with the insulation film 131 formed on.
Then, an opening 137 is formed in the insulation film 135 and the insulation film 134 down to the base layer 132.
An n type emitter layer 136 is formed, connected to the base layer 132 through the opening 137.
Then, as shown in FIG. 11C, the polycrystal silicon layer 125 in the region except for the outgoing base electrode 126 is etched by photolithography.
Thus, the proposed HBT is fabricated.
Such HBT using silicon germanium is expected to realize high-speed operations of a 100 GHz cut-off frequency fT.
However, in the proposed HBT described above, the boron is often diffused out of the base layer 132 by heat processing made after the emitter layer 136 has been formed. When the boron is diffused out of the base layer 132, a transit time of the carriers in the base layer 132 becomes longer, which leads to a lower cut-off frequency fT.
In the proposed HBT, the growth of the base layer 132 immediately below the outgoing base electrode 126 is very unstable. Often voids occur. Accordingly, in the proposed HBT, the connection between the base layer 132 and the outgoing base electrode 126 is unstable, and a parasitic resistance between the base layer 132 and the outgoing base electrode 126 often becomes high. The parasitic resistance increase between the base layer 132 and the outgoing base electrode 126 results in increased noises.
Here, in order to make a parasitic resistance between the base layer 132 and the outgoing base electrode 126 low, it will be one means to side-etch much of the pad oxide film 118 immediately below the outgoing base electrode 126. However, it results in an increased parasitic capacitance between the base layer 132 and the collector layer 114, which much hinders high-speed operation of the HBT.
Under such circumstances, an HBT which can realize high-speed operation and can reduce noises is much expected.